You've probably chosen the wrong A level subjects for you or something.
I was never an academic but forced myself to work extremely hard from the age of 11 to get streamed into the higher ability groups at GCSE, especially for maths, science subjects etc. achieving A*-B. However I found A Levels in Physics, Maths, etc a completely different ball game. I really wanted to do Business studies, Economics, Geography or History! I didn't complete the courses and went into the world of work at 17. Could you re-course and start year 1 again?
20 months to complete your A levels would be a sound investment. Its your last chance at FREE education in the UK. As already stated, you will need them for FFP, CTC, University, etc.
Will having no A Levels stop you from getting a flying job later? No, not necessarily. I was invited to interview by BA with just GCSE's (but as a Direct Entry Pilot with 7 years on Boeing). However, some expat jobs require college/university level education (Korean for 777/747/A330).
In todays industry I think they are pretty much essential for entry in commercial aviation training and thereafter to achieve your first job as a cadet pilot. A levels with average or poor grading will serve you better than no qualifications at all.